Recently, with the technology development and the increasing demand for mobile devices, the demand for secondary batteries as a source of energy has also increased. Among secondary batteries, lithium secondary batteries with high energy density and voltage, a long cycle life and a low self-discharge rate have been developed. In addition, with the growth of the market of devices employing high capacity batteries, the demand for high capacity batteries is expanding, and there is a need for a higher capacity electrode design to manufacture lithium secondary batteries with high energy density, high output and high discharging voltage as a source of power for these devices.
However, due to the volume and weight of an electrode layer, these developed high capacity batteries reduce in energy density of all battery cells and experience non-uniformity of electrochemical reaction in an electrode active material layer.
When current or voltage is applied to a secondary battery, a concentration gradient of lithium ions takes place in the electrode, and this concentration gradient phenomenon in the electrode becomes severer with the increasing thickness of the electrode active material layer, causing deterioration in boosting charge characteristics. In addition, an electrochemical reaction preemptively occurs on the surface layer of the electrode that contacts with an electrolyte solution, and as a consequence, lithium metal is deposited on the surface layer while boosting charge.
As described above, because reactivity of lithium ions in the electrode active material layer is not uniform due to the thickness of the active material layer, a high-loading electrode is used to improve the battery performance, but nevertheless, in effect, only part of electrode active material participates in the battery reaction, so the battery performance is not improved as much as expected from the high-loading electrode.
Therefore, if quantitative measurements of electrochemical reaction non-uniformity in the thickness-wise direction of the electrode during the operation of the battery is possible, such measurements could be used as data in implementing design for optimization when applying a new material for enhanced boosting charge.